1. Field of the Invention
This invention relates to a substrate, a semiconductor device, and a method of manufacturing the same, and more particularly to a substrate in which at least a part of a front surface is composed of silicon carbide, a semiconductor device, and a method of manufacturing the same.
2. Description of the Background Art
A substrate in which at least a part of a front surface is composed of silicon carbide, such as a silicon carbide single crystal substrate, has conventionally been known. Since such silicon carbide is higher in thermal conductivity than a nitride semiconductor such as gallium nitride (GaN), a substrate composed of silicon carbide is expected as a material for a power device for controlling a high voltage and a high current. For example, United States Patent Publication No. 2006/0225645 (hereinafter referred to as PTL 1) discloses a silicon carbide substrate having a diameter of 3 inches, for which a value for warp or TTV (Total Thickness Variations) is defined in order to prevent film quality of an obtained epitaxial film from deteriorating due to non-uniform temperature distribution at the time of forming an epitaxial film on a front surface thereof. In addition, WO2010/119792 (hereinafter referred to as PTL 2) discloses definition of a characteristic specifying a shape, such as warp or bow of a substrate, and setting a value for surface roughness Ra on a front surface side of the substrate to 1 nm or smaller and setting a value for surface roughness Ra on a back surface side of the substrate to 100 nm or smaller. PTL 2 defines surface roughness as above, in order to ensure a prescribed shape of a substrate when a thin film is formed on the front surface of the substrate.
PTL 1 described above does not particularly mention surface roughness of a substrate. Though PTL 2 defines surface roughness of a front surface and a back surface of the substrate in order to ensure a prescribed shape of the substrate, it does not mention either relation between surface roughness of the substrate (in particular, surface roughness on the back surface side) and film quality of an epitaxial film or the like formed on the front surface of the substrate.
As a result of the inventor's dedicated studies, however, the inventor has found that, when surface roughness in particular on a back surface side of a substrate is great, during heating treatment in a step of growing an epitaxial film on a front surface of the substrate, a state of contact between a susceptor on which the substrate is carried and the back surface of the substrate varies and consequently temperature distribution is produced in the substrate. Such temperature distribution may adversely affect film quality of the formed epitaxial film.
In addition, in steps of forming an epitaxial film on the front surface of the substrate and forming a semiconductor element on the epitaxial film, the back surface of the substrate may be stuck by vacuum. If surface roughness at the back surface is great during such sticking by vacuum, the substrate cannot reliably be stuck in some cases. Consequently, defects may be produced in the step of forming a semiconductor element.
Furthermore, when surface roughness at the back surface of the substrate is great, a crystal defect may develop from the back surface side of the substrate during heating treatment for forming an epitaxial film and the substrate may warp. Then, in the case where a substrate is increased in size, an absolute value of an amount of warp of the substrate becomes great, which may result in a factor for causing defects in a step of forming an epitaxial film or forming an element.